ok i dont think your really understanding what many are trying to explain to you..your mind seems made up and of course thats your choice. While there are some very educated people in here trying to offer you some suggestions based on personal experiences for some reason..your not willing to trust their judgement.
You first asked about a single bird and what to do about its seemingly lack of perkiness so to say...and you stated you wanted to get her a friend thinking that was the best solution while many tried to discourage that for a very good reason and yet explain to you a way to handle your problem. Now you may simply compound that problem two fold as you still want to get another bird and now mirrors for both.
Im going to try to state this in a way that maybe more understandable for you..if you feel one bird is lonely its NOT MEAN to put a parakeet mirror in for her where she can interact with it rather than get another bird which may cause jealousy and exhibit dominance behaviour which also is not going to help. Also another bird may stimulate breeding mode which could potentially lead to even more serious problems until you decide you want to learn more about breeding and ALL the complications that could arise and the responsibility of having to potentially take care of babies. You by interacting with her and being patient as it takes a lot of time sometimes to train your friend would be a very suitable mate/friend to her to keep her happy, and not compound a stituation. If you dont have time to devote to this thats ok also as many with busy schedules dont..but as stated her by several and them giving very valid reasons also as to why they suggest certain things, a simple parakeet mirror and some patience would solve the problem you have been describing..either way a mirror or you interacting she will bond with one or the other and be very content without getting another bird till you become more familar with the mind workings of birds. Which by the way ..we ALL have had to learn through time but a lot of us didnt have the advantage that you do of getting advice from people who have gone through what your experiencing through this forum...I urge you to give all their suggestions a FAIR try and give it time. I think you will do just fine as you do seem to be a caring person wanting to do the best thing for your bird..which you will do by giving some of these suggestions a chance.